Traction drives for vehicles capable of providing either continuously variable (CVT) or infinitely variable (IVT) ratio transmissions have long been recognized as advantageous for many potent applications, such as those in automotive, motorcycle, recreational vehicles, heavy machinery, agricultural, robotic, power generation, oil and gas, HVAC and other industries and settings.
Many, if not most, traction type CVTs or IVTs are similar in the way that they utilize rollers contacting other rotating surfaces (and are met particularly in so-called toroidal, half toroidal, conical, ball type and other transmissions) where rollers transfer power from one rotating body to another while roller's rotational axis can tilt or move thus providing that the contact points at the rotating input and the rotating output are of different distance or radii from rotating bodies axes, thus providing a variable ratio.
In order to achieve IVT mode such devices will typically employ so-called ‘power loop’ configuration where the unidirectional rotational speed of the output of CVT will be subtracted from the rotational speed of the reference speed of the input. These configurations generally employ differential mechanisms or planetary gear sets to achieve such subtraction therefore reducing the overall efficiency of such devices due to high internal losses the ‘power loop’ systems are known for.
The rollers, except in the configurations defined by a so-called ‘Dual Roller Arrangement’ (DRA) WO2011113149 (A1) by Okulov, while permitting the input and output bodies to rotate at different speeds, encounter a substantial spin at traction points which reduce the available traction (in traction fluid based configurations) or cause an excessive wear of the mating rotating bodies in metal-to-metal or other material direct contact devices.
The half toroidal transmissions where spin at rolling contact points is significantly reduced still suffer from the relatively low efficiency, particularly due to high frictional losses in bearings supporting the rollers and withstanding a substantial portion of the entire clamping force.
Because all rollers in most types of prior art transmissions are typically clamped between fairly rigid surfaces, there is a difficulty providing an equal distribution of the clamping forces (necessary for reliable traction in a multi-roller environment) amongst them. The contact points having different clamping force would behave differently in a sense of percentage of the slip experienced under traction load and if the amounts of slip at all points are not harmonized, it will causing appearance of significant internal forces and losses, reducing the overall efficiency. In practice, this disadvantage calls for the number of rollers to be reduced to 3-4 per one pair of input-output bodies (or per one cavity of a toroidal transmission), which limits power density of such transmissions and the maximum torque they can handle.
It is also well known in the trade and practice that traction devices with high spin will exhibit more forgiving behavior and more plasticity (at the cost of reduced traction coefficient compared to its maximum available value at zero spin) compared with the low spin devices, which are sometimes unpredictable in their performance and reliability due to low deformability of traction points.
Numerous examples of CVTs configurations employing spherical bodies driven by rollers is evidenced from U.S. Pat. No. 2,139,635 by House; U.S. Pat. No. 5,923,139 and US patent application 2008/0081728 by Faulring et. A1. These devices exhibit low efficiency and low torque capability due to high spin driving and steering rollers have during manipulation of spherical body.
Another approach to CVTs configurations evidenced from numerous US patents, namely U.S. Pat. No. 1,141,508 by Weiss; U.S. Pat. No. 1,146,982 by Weiss; U.S. Pat. No. 1,469,061 by Weiss; U.S. Pat. No. 1,537,515 by Weiss; U.S. Pat. No. 1,541,882 by Weiss; U.S. Pat. No. 1,728,383 by Weiss; U.S. Pat. No. 2,020,123 by Pollard; U.S. Pat. No. 2,128,088 by Hanft; U.S. Pat. No. 2,682,776 by Morgan; U.S. Pat. No. 2,959,971 by Salomon; U.S. Pat. Nos. 3,826,157; 4,487,086; 4,964,316 by Perkins; U.S. Pat. Nos. 7,207,918; 7,207,918 by Shimazu and U.S. Pat. No. 7,594,870 by Ferrar and is employing a carrier with rollers mounted on it and rotatable in the direction perpendicular to the rotational plane of the carrier and being in a frictional rolling contact with a rotatable spherical surface. The relative angle between the rotational plane of the carrier and the sphere's rotational axis is defining the transmission ratio. At carrier position perpendicular to the rotational axis of the sphere the ratio of the transmission is approximately 1:1.
Over the years, numerous improvements to such CVTs have been disclosed. Still, the deficiency of such transmissions is their inability to provide neutral ratio or ‘geared neutral’, i.e. to achieve zero output rotational speed when the input is rotating as well as relatively low range of the ratios that can be achieved limited by geometrical constrains of the sphere-carrier pivoting mechanism. Still, another disadvantage of these inventions is low efficiency due to different behaviour of the rollers working in parallel, but sharing different load.
The phenomena of different behaviour of rollers working in parallel under different load conditions can be illustrated as follows:
Let's picture a straight road with two identical cars moving forward, side by side and fighting against strong cross winds in order to maintain its straight moving direction (FIG. 1). Car #1 is experiencing the strongest lateral force from the cross wind and its driver has to apply the correction steering angle θ1 to its left in order to maintain the movement in a straight line. Car #2 is enjoying a shadow from the cross wind provided by Car #1 and has to apply lesser correction steering angle θ2 to its left in order to maintain the movement in a straight line. By doing so the both cars remain moving in parallel and even if they would have been linked together it would not cause any parasitic forces between them. In the systems described in the prior art relevant to present invention, the steering angles of all rollers remain the same regardless of the condition of loading. That disadvantage is manifested by significant losses which origin will be illustrated in more detail later. FIG. 2 provides insight on how the parasitic force between linked cars can lower the efficiency of the system.
Going back to transmissions described here above and incorporating rollers mounted on a carrier and pre-aligned in certain direction it is now clear that unless their steering angles are augmented according to the lateral loads they experience, they would produce unnecessary losses.
There are a variety of vehicles known in robotics which utilize a steerable wheels rotating on a carrier and in contact with ground and where controlled steering can produce omnidirectional movement of such vehicles (U.S. Patents: U.S. Pat. No. 3,789,947 by Blumrich; U.S. Pat. Nos. 5,927,423; 5,927,423 by Wada et al.; U.S. Pat. No. 6,491,127 by Holmberg et al.; and U.S. Pat. No. 6,810,976 by Rohrs as well as PCT case WO2003/091600 by Rohrs. These systems fail to provide a solution for traction drives, particularly ones utilizing non contact environment with power transferred through the film of traction fluid. Neither of these prior art means is utilizing an augmentation to the steering direction of rollers working in parallel mode mentioned above, which is necessary to optimize the efficiency. Similar systems are also well known from the prior art related to ship propulsions systems (Voith Schneider Propeller; www.voithturbo.de) and some experimental windmills (Vertical Mills).
Therefore, there exists a need for an improved infinitely variable transmission that would provide high efficiency over whole range of ratios, be cost effective, compact, reliable and usable for automotive and other industries mass applications.